Repurposing rosmarinic acid as an anti-colorectal cancer agent through bolstering T cell anti-tumor immunity by enhancing activation of MEK1-mediated TCR signaling.

Activation of the T cell receptor (TCR) complex is fundamental to initiating adaptive immune responses, particularly in CD8⁺ cytotoxic T lymphocytes that mediate anti-tumor immunity. However, the immunosuppressive tumor microenvironment often impairs TCR signaling, limiting the efficacy of T cell-based cancer immunotherapies. Here, we report the identification of rosmarinic acid (RA), a naturally occurring polyphenolic compound, as a potent small molecule enhancer of TCR signaling. RA significantly augments IL-2 and IFN-γ production, promotes T cell proliferation, and enhances cytotoxicity of CD8⁺ T cells in vitro. Mechanistically, RA directly binds to MEK1, a key kinase in the MAPK/ERK pathway, with high affinity, leading to reduced MEK1 phosphorylation and downstream signaling activation. Transcriptomic and metabolic profiling of RA-treated CD8⁺ T cells revealed upregulation of genes associated with TCR signaling, calcium flux, PPAR signaling, and oxidative phosphorylation, indicating a broad remodeling of T cell effector function and metabolism. In vivo, RA treatment significantly suppressed tumor growth, increased tumor-infiltrating lymphocyte (TIL) frequency, and improved survival in MC38 tumor-bearing mice. These effects were abrogated upon CD8⁺ T cell depletion, confirming their central role. Furthermore, RA synergized with anti-PD-1 therapy and enhanced the efficacy of adoptively transferred OT-I T cells in colorectal tumor-bearing hosts. Collectively, our findings reveal RA as a novel immunomodulatory agent that boosts CD8⁺ T cell responses via MEK1-mediated TCR signaling enhancement, providing a promising strategy for drug repurposing in cancer immunotherapy.